Ventana software support forum

Yes, as Bob says, there are easy ways to do this (they are in the SD texts). Ill go into some detail. > O1(n+1) + O2(n+1) <= S(n) First point is that the above is dimensionally invalid. S must be in [units] while O1 ands O2 are in [units/week] (or other time unit. [ ] denotes the dimensions of). In ...

Folks, Over the last few years with this excellent discussion group Ive noticed two types of message. One is from an enquirer who wishes to find out something about system dynamics. The other is a discussant who wants to contribute to a debate. Fair enough, but its worth pointing out that this discu...

Folks, I noticed at the Palermo conference that some people were reading books relevant to Afghanistan and terrorism and I wondered if people wanted to share their reading lists. Mine is Malise Ruthvnes A Fury For God, Granta Publications, ISBN 1 86207 5409, 2002. Its mainly about the psychological ...

Colleagues, Im grateful to Lazaros, Bob, and others who have written to me privately, for understanding the point I made about discretion. I had intended to reply to Raymond Joseph but have only just found time to do so. A few points: 1 A good SD model analyses policies and, were an analyst to find ...

Id guess that youd have a hard time doing this in SD. Its a very standard type of problem, usually handled with discrete event methods. The interesting feature is the rules to be used to select work at each batch stage, such as shortest job first, oldest job first and so on. Outputs are usually as d...

This is an interesting question and I hope that it gets a good response. I know of no explicit research on this topic, though it deserves some. Group model building must help, as long as there is time in the group sessions to debate causal reasoning, which may or may not be the case, but still leave...

Folks, Ive been away in Australia for a month so Ive missed much of this correspondence but, as usual, Jay has got it right. The unit in which TIME is measured has nothing at all to do with whether or not SD is a suitable modelling method. Ive heard somewhere that someone is modelling geological pro...

Yaman has hit the point exactly. To say that all we need is SD tools is absurd; the object of the whole exercise is to help problem-owners understand and solve their problems, it is not to do SD. We sometimes go even further and suggest that SD is the only analytical methodology. It isnt. There are ...

Alexander (and others),

Yes, thats a fair comment but it still leaves open the question of how do
you know it is the right direction. Research is desperately needed into
this kind of topic.

Geoff
From: "geoff coyle" <geoff.coyle@btinternet.com>

Surely this was all laid out in Jays book 40 years ago. The order of a system dynamics model is the number of level variables it contains, including the implicit levels within functions such as DELAY3. Regards, Geoff Professor R G Coyle, Consultant in System Dynamics and Strategic Modelling, Telepho...

Folks, This important question seems to come up from time to time and gets many excellent comments, such as Jays own remarks. My take on it is that, of course, one cannot omit from an analysis variables which cannot easily be quantified. If a variable such as integrity (or perceived integrity) is re...

Its also easy to embed discrete events within an SD model (where the purpose of the model calls for it). I can provide reference if anyone wants them. Regards, Geoff From: "geoff coyle" <geoff.coyle@btinternet.com> Professor Geoff Coyle Consultant in System Dynamics and Strategic Analysis,...

Folks, Im afraid that Alexander Leus has somewhat missed the point I was trying to express which is the degree of uncertainty introduced into a model by multiplying factors which are inherently uncertain. Put simply, if we do not know the real values of parameters we do not know if the model output ...

Folks, Some of you will have noticed concerns Ive expressed in the past months about erroneous models, that is to say, models in which the equations are technically incorrect or which calculate incorrectly for a number of other reasons. Last week, I mentioned rate-dependent-rates and Ive now receive...

Folks,

I absolutely agree that it is vital to know something of the rest of OR if
one is to do good SD.

I am also appalled at this ridiculous comment in a UK government paper. How
do we access it and correct it?

Geoff
From: "geoff coyle" <geoff.coyle@btinternet.com>

Folks, The discussion about text-based and iconic SD packages has been = interesting. A few days ago I got one on this topic but, unhappily Id = been given a virus and, when deleting that, I also deleted the message. = I cant, therefore, recall his name but I do remember the gist of the = message. H...

Bill, Ill be glad to know what you think. One thing which worries me is the uncertainties in these multipliers. Lets say we have three: X=A*B*C and, as you point out, if all are 0.5, the net effect is 0.125. However, the data for these things is usually a bit suspect and each might actually be somew...

Not sure that I agree with Bills argument. The need seems to me to be for SIMPLICITY and CLARITY, which is what the text-based softwares provide. Bills mention of code development tools is, to my, somewhat off the mark as the task is to model, not to write code. Someone mentioned icons on the screen...

We seem to get a lot of these requests and its not a bad idea to start by looking at the SD literature. Theres a refereed journal called System Dynamics Review, try Vol. 13, No 3, Fall 1997! You might also look at a more obscure journal Small Wars and Insurgencies, Vol. 7, No 3, winter 1996. Geoff C...

Bill,

An interesting question. I tried to deal with it in Qualitative and
quantitative modelling in system dynamics: some research questions, System
Dynamics Review, 16, 225-244, 2000.

Hope that helps

Geoff
From: "geoff coyle" <geoff.coyle@btinternet.com>

SD and Markov is, has been said, a very tricky problem but I dont see why one would want to do it. My (limited no doubt) grasp of Markov is that there is no causation. States reflect, absorb etc at random according to their type. Its much closer to discrete event simulation and SD is fundamentally a...

Dear All, A reply to Petrides, which may be of general interest. What an interesting comment though I suppose that I am biased as I directed the development of DYSMAP (Dysmap2 is a version developed by Salford University under the direction of Brian Dangerfield) and I enhanced DYSMAP to turn it into...

Dear All, The idea of SD certification is a very interesting issue but it might be fraught with difficulty. One precedent is that the Operational Research Society (which has around 5000 members world-wide) argued for at least 15 years over the establishment of the Fellowship of OR - equivalent to ce...

Some time ago D C Lane of the London School of Economics placed a message here saying that he has written a paper on what he called modes of discourse between DES and SD. He invited people to contact him if they wished to receive a copy of his paper. Probably some of you (or many of you) did ask for...

Folks, Heres a small further contribution to the qualitative versus quantitative debate. Some of you may remember C P Snow (later Lord Snow). For those who do not, he came from humble origins, read physics at Cambridge and (despite that handicap) went on to become very great and very good in the Bri...